--- a/testing/mochitest/runtests.py
+++ b/testing/mochitest/runtests.py
@@ -2691,18 +2691,21 @@ toolbar#nav-bar {
                 self.harness.countpass += val
             elif "Failed:" in line:
                 self.harness.countfail += val
             elif "Todo:" in line:
                 self.harness.counttodo += val
             return message
 
         def fix_stack(self, message):
-            if message['action'] == 'log' and self.stackFixerFunction:
-                message['message'] = self.stackFixerFunction(message['message'])
+            if self.stackFixerFunction:
+                if message['action'] == 'log':
+                    message['message'] = self.stackFixerFunction(message['message'])
+                elif message['action'] == 'process_output':
+                    message['data'] = self.stackFixerFunction(message['data'])
             return message
 
         def record_last_test(self, message):
             """record last test on harness"""
             if message['action'] == 'test_start':
                 self.harness.lastTestSeen = message['test']
             return message